The present invention relates to drills having twisted flutes in the shank and to a method of and an apparatus for grinding the drill.
The problems encountered with the so-called twist drill having twisted flutes in its shank are generally concerned with cutting ability and chip dischargeability. If the drill is caused to cut workpieces more powerfully, a correspondingly increased amount of chips are produced. When the chip discharging flutes are not sufficiently large, chips will bite the work inner surface defining the bore formed or clog up the flutes to make the drill unable to continue its operation. The chip discharging flutes, if sufficiently large-sized, correspondingly reduce the rigidity of the shank, making the drill unable to perform a powerful cutting operation.
Usual drills are formed with a chisel positioned approximately at the center of rotation and having a large negative rake angle, so that this portion almost effects a pushing action rather than cutting, consequently offering a large proportion of resistance to the cutting operation of the drill.
To improve the low cutting ability of the drill at its central portion, Transactions of the ASME (February 1957), for example, proposes providing central cutting edge portions at the position where the chisel is formed. Stated more specifically with reference to FIGS. 10 and 11, a drill 1a is provided at one end with a pair of cutting edges 2a arranged symmetrically about a point. The cutting edge 2a comprises a central edge portion 21 and an outer edge portion 22 each of which is straight. The central edge portion 21 has a cutting face 25 parallel to the axis of the drill 1a.
A first land portion 27 is formed on the rear side of the flank 23 of the edge portions 21, 22 with respect to the direction of rotation, and a second land portion 28 is formed on the rear side of the land portion 27 with respect to the direction of rotation. Since the first land portion 27 is subjected to the resistance offered to the edge portions 21, 22 during cutting, the first land portion 27 has a relatively gentle inclination (close to a direction perpendicular to the drill axis), while the second land portion 28 has a steep inclination (close to the axial direction of the drill) so that the cutting face 25 can be formed.
Thus, the gently sloping land portion 27 is formed to assure the cutting edge portions 21, 22 of rigidity, whereas if the overall land portion is given such an inclination, the central edge portion 21 has a very small cutting face and fails to cut workpieces satisfactorily, so that the steep land portion 28 is formed beyond a ridge line 28a to provide the large cutting face 25. Indicated at 3 are chip discharging flutes formed helically in the shank.
With the above construction, the furrow for discharging therethrough the chip produced by the central edge portion 21 is formed between the cutting face 25 and the ridge line 28a at the boundary between the first and second land portions 27, 28. The furrow has a small opening angle 8 of up to 90 degrees when seen from below and is therefore low in chip dischargeability. Thus, the drill central portion still remains to be fully improved in cutting ability. Moreover, the second land portion 28 is slanted steeply (nearly along the axis of the drill) to provide the cutting face 25, with the result that the outer periphery is greatly cut out to decrease the rigidity of the shank.
Although attempts are made to afford improved cutting ability by minimizing the width of the chisel, for example, by web thinning, the methods heretofore used have the drawback of failing to achieve improvements in the cutting ability of drills because the chips produced at the central portion are not satisfactorily dischargeable or because the central edge portions are insufficient in strength to render the drill unusable for a powerful cutting operation.